Field of the Invention
The invention concerns a method to optimize a pulse sequence for a magnetic resonance sequence. Moreover, the invention concerns a method to operate a magnetic resonance system using such an optimized pulse sequence, as well as a pulse sequence optimization device for a magnetic resonance system in order to implement such a method.
Description of the Prior Art
In a magnetic resonance system (also called a magnetic resonance tomography system), the body to be examined is typically exposed to a relatively high basic magnetic field—for example of 1, 5, 3 or 7 Tesla—by operation of a basic field magnet system, which often contains superconducting coils. A magnetic field gradient is additionally applied with the aid of a gradient system. Radio-frequency excitation signals (RF signals) are then emitted via a radio-frequency transmission system, which leads to the situation that the nuclear spins of specific atoms excited to resonance by this radio-frequency field are flipped (deflected) by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals (what are known as magnetic resonance signals) are radiated that are received by means of suitable radio-frequency antennas and then are processed further. Finally, the desired image data can be reconstructed from the raw data acquired in such a manner.
For a defined measurement, a defined pulse sequence is emitted that is composed of a series of radio-frequency pulses (in particular excitation pulses and refocusing pulses as well as gradient switchings emitted in coordination therewith) in different spatial directions. Readout windows must be set to match these chronologically. The readout windows predetermine the time periods in which the induced magnetic resonance signals are received. The timing within the magnetic resonance sequence—i.e. at which time intervals which pulses follow one another—is significant to the imaging. A number of the control parameters are normally defined in what is known as a measurement protocol, which is created in advance and retrieved (for example from a memory) for a specific measurement, and can be modified on site if necessary by the operator, who can provide additional control parameters such as (for example) a defined slice interval of a stack of slices to be measured, a slice thickness etc. A pulse sequence (also designated as a measurement sequence) is then calculated on the basis of all of these control parameters.
The gradient pulses are defined by their gradient amplitude, gradient pulse duration and edge steepness, i.e., the first derivative of the pulse shape dG/dt of the gradient pulses (typically designated as a “slew rate”). An additional important gradient pulse value is the gradient pulse moment (also shortened to “moment”), which is defined by the integral of the gradient amplitude over time.
During a pulse sequence, switching between the magnetic gradient coils from which the gradient pulses are emitted takes place often and quickly. Since the time specifications within a pulse sequence are for the most part very strict, and in addition to this the total duration of a pulse sequence (which determines the total duration of an MRT examination) must be kept as short as possible, at present gradient strengths around 40 mT/m and slew rates of up to 200 mT/m/ms must be achieved. In particular, such a high edge steepness contributes to noise development during the switching of the gradients. Eddy currents with other components of the magnetic resonance tomograph (in particular the radio-frequency shield) are one reason for these noise exposures. In addition to this, steep edges of the gradients lead to a higher power consumption and additionally place higher demands on the gradient coils and the additional hardware. The rapidly changing gradient fields lead to distortions and oscillations in the gradient coils, and to the transfer of these energies to the housing. A high helium boil-off of the vessel that contains the superconducting coils can additionally occur due to heating of the coils and the additional components.
In order to reduce the noise exposure, various solutions have been proposed in the design of the hardware, for example potting or vacuum sealing of the gradient coils.
In DE 10 2013 202 559 a method is described in that a finished pulse sequence which should be sent to the scanner of the magnetic resonance system is analyzed in order to determine the time interval within the pulse sequence that is to be optimized with regard to a gradient curve, or a gradient pulse. All original commands that are sent to the scanner are captured, then initially examined for optimizable regions. These regions are optimized, and only then does the forwarding to the scanner take place. The optimization preferably takes place with the use of a spline interpolation that satisfies defined boundary conditions, among other things a defined gradient moment, and/or a defined amplitude at predetermined node points, in particular at a start point in time of the respective interval and at an end point in time of the respective interval. An optimally smooth gradient curve with rounded edges is generated via the spline interpolation. However, the calculation of such a spline interpolation takes a relatively long time (especially in the case of longer gradient intervals), such that this could lead to the termination of the measurement if only a small amount of time is available for the optimization. This can be problematic given real-time applications.